Magnetic sensor constructed by using superconducting quantum interference device (hereinafter referred to as SQUID) is a currently known magnetic sensor with the highest sensitivity. It is widely used in the field of detecting weak magnetic field such as biological magnetic field, earth magnetic field abnormity, extremely low field nuclear magnetic resonance, the magnetic field detection sensitivity thereof can be measured by fT (10−15 tesla). It is an important magnetic sensor in the field of extremely weak magnetic field detection and research. Signal response of SQUID device can be varied from DC (0 Hz) to GHz and has characteristics of high speed and high bandwidth. Furthermore, since SQUID chip is prepared by using microelectronic technology, it has unique advantages of small size and integration. It is widely used in multi channel, the high resolution detection system, such as in the magneto cardiogram instrument using 64 channels and magneto encephalography instrument using more than 200 channels of SQUID.
Direct-current superconducting quantum interference device (referred to as DC SQUID) is constructed by two superconducting Josephson junction in parallel connection to form a superconducting ring. Drawing wire from the two terminals of Josephson junction forms a two terminal element. When a bias current is applied across SQUID, the voltage across the SQUID will vary as a function of magnitude of external magnetic flux detected by superconducting ring due to superconducting quantum effect and Josephson Effect. Since the output voltage of SQUID assumes a non-linear relation with the detected magnetic flux thereof, people cannot obtain the magnitude of magnetic flux by directly measure of voltage across SQUID. Therefore, practical SQUID magnetic sensor is implemented by a flux-locked loop (hereinafter referred to as FLL) comprising a SQUID device and an amplifier circuit. Such flux-locked loop is referred to as SQUID readout circuit.
A typical SQUID flux-locked loop introduced in document [D. Drung and M. Mück, The SQUID Handbook, vol. 1, J. Clarke and A. I. Braginski, Ed. Weinheim: Wiley-VCH, pp. 128-155, 2006.] is shown in FIG. 1. Firstly, a bias current Ib is applied to SQUID to achieve maximal flux-voltage transfer rate. The SQUID voltage signal is delivered to a preamplifier to amplify it. Meanwhile, bias voltage Vb is adjusted to keep the DC component of the output voltage of SQUID to be zero when SQUID has maximal flux-voltage transfer rate. At this time, the current value status of the bias current Ib, the bias voltage Vb and the applied fluxΦa are referred to as operating point. The output of the preamplifier is delivered to integrator to integrate, the output of the integrator drives a feedback resistor to inject feedback current into a feedback coil, mutual inductance by the feedback coil and SQUID generates feedback flux coupled to SQUID.
The operation principle of the flux-locked loop is: provided the flux-locked loop maintains a stable operating point for SQUID, when the tested flux has a variation Δ Φ, SQUID generates a voltage variation ΔV deviating from the operating point, the voltage variation ΔV, after being amplified by the preamplifier, is delivered to the integrator to integrate, the output voltage of the integrator is adjusted. The feedback flux is adjusted by the feedback resistor and the feedback coil to counteract the above mentioned flux variation, making the voltage of SQUID input to the integrator at the operating point to be zero, thus the integrator stops integrating and the loop becomes stable. The stable status of the operating point is referred to as readout circuit lock. Under the lock state, the output voltage Vf of the flux-locked loop will be in proportion to external magnetic flux variation sensed by SQUID [D. Drung “High-Tc and low-Tc dc SQUID electronics” Supercond. Sci. Technol. 16 1320-1336 (2003).], i.e. Vf=ΔΦ·Rf/Mf, wherein ΔΦ is magnetic flux variation sensed by SQUID, Rf is feedback resistance, Mf is mutual inductance by the feedback coil and SQUID.
It thus can be seen, SQUID magnetic sensor is a negative feedback based flux-locked loop. Since the output signal from SQUID is relative small, the conventional negative feedback circuit is based on PID (P proportion, I integral, D differential) principle, i.e. firstly using a proportional amplifier to amplify the output signal from SQUID, then using an integrator circuit for eliminating error to drive flux feedback circuit. The function of the integrator is to make the output voltage of the SQUID temporarily departure from the operating point when the input flux changes, then integrator starts to integrate and modulate the output voltage, until the feedback flux driven by the output voltage counteracts the change of the input flux, consequently, the flux-locked loop resume to stable balance. Therefore, the integrator in the flux-locked loop is used to accumulate error, modulate output voltage until the error is eliminated.
That is to say, the conventional SQUID flux-locked loop typically uses preamplifier to perform low-noise amplification for weak voltage signal of SQUID and then the amplified signal is delivered to the integrator to perform integral feedback. Therefore, the conventional SQUID flux-locked loop is constituted by at least one preamplifier and one integrator cascade d circuit connected with feedback resistor and feedback coil. In practical application, the SQUID flux-locked loop employing integrator has several drawbacks:
1) Limited bandwidth. The readout circuit employing integrator uses more than two amplifier stages, which increases time-delay of the loop signal and makes high frequency signal have phase-shift, thus causing loop oscillation [D. Drung and M. Mück, The SQUID Handbook, vol. I, J. Clarke and A. 1. Braginski, Ed. Weinheim: Wiley-VCH, pp. 128-155, 2006.]. For this reason, the integral capacitance must be increased to eliminate oscillation and realize stable operation of the flux-locked loop. As the integral capacitance increases, the bandwidth of the flux-locked loop narrows. Thus the tracking speed of the flux-locked loop is slow and cannot follow the change of the input signal, which causes the lock failure will easily occur and the system cannot operate properly.
2) Limited slew-rate. The output of the integrated circuit is the result of the time integral of the input signal. When the detected signal changes suddenly, the integrator cannot respond instantly, thus causing a limited slew-rate of the sensor voltage output. It is difficult to meet the requirements for detecting high slew-rate magnetic field.
3) It is needed to use at least two operational amplifiers and peripheral circuits which circuits are complex and cause large power consumption.
Therefore, the above readout circuit employing integrator limits the performance of SQUID magnetic sensor in terms of the bandwidth and slew-rate. Furthermore, the Multi-channel system presents higher requirements on miniaturization for SQUID magnetic sensor. Therefore, simplifying the design of SQUID readout circuit is of great significance and practical importance for the application of multi channel SQUID. The present invention seeks to settle these problems.